Japanese Patent Publication JP2002-352970 A discloses a lamp ballast for a high intensity discharge lamp which includes a lamp discriminating means so as not to supply an excessive power upon being accidentally connected to an incandescent lamp such as a halogen lamp. The lamp ballast includes a DC-DC converter providing a variable DC voltage, an inverter converting the output voltage from the DC-DC converter into the AC power to be applied to a lamp load, a starting circuit for generating a high voltage pulse out of the output voltage from the inverter and applying the high voltage pulse to the lamp load, and a control circuit for varying the output voltage from the DC-DC converter. The lamp discriminating means is configured to cause the DC-DC converter to generate a low detection voltage before generating the high voltage pulse, determine whether the lamp load is the discharge lamp or the incandescent lamp based on the detection voltage, and output thus determined result to the control circuit. When the result identifies the discharge lamp, the control circuit operates the DC-DC converter to increase its output up to a normal voltage, thereby allowing the inverter and the starting circuit to generate the high voltage pulse for starting the discharge lamp, and subsequently controlling the inverter to provide the AC power for lighting the discharge lamp. When, on the other hand, the incandescent lamp is identified, the control circuit causes the DC-DC converter to lower its output voltage from the normal voltage, thereby avoiding an excessive power from being supplied to the incandescent lamp.
However, the starting circuit is also activated under a condition where the incandescent lamp is connected to the lamp ballast, and then applies the high voltage pulse to the incandescent lamp. This causes the incandescent lamp to suffer from an undue electric stress. The undue electric stress shortens a life of the incandescent lamp.